A Clockwork Orange

In a futuristic Britain, a gang of teenagers go on the rampage every night, beating and raping helpless victims. After one of the boys quells an uprising in the gang, they knock him out and leave him for the police to find. He agrees to try "aversion therapy" to shorten his jail sentence.

When he is eventually let out, he hates violence, but the rest of his gang members are still after him.